The present invention relates to thermally conductive adhesive compositions, and more particularly, to low viscosity thermally conductive adhesive compositions filled with spherical thermally conductive particles, which find use in electronic applications.
Thermally conductive adhesive compositions are useful in many applications""such as in electronics, ceramics, paints and coatings, where an effective thermal contact is sought, such as between a thermocouple probe and a heat source, or between two bolted metal parts having an air gap across which heat is transferred.
Thermally conductive compositions are particularly useful to encapsulate electronic components, such as semiconductors. These compositions protect the electronic components from corrosion and serve as a conduit to remove heat to a heat sink during normal operation. The compositions enhance the performance of electronic parts and have been particularly helpful in their miniaturization.
Recent advances in semiconductor technology have led to chips with more power and higher densities, which are challenging the capabilities of the currently available thermally conductive compositions. To increase the thermal conductivity, conventional thermally conductive compositions typically contain higher concentrations of thermally conductive particles, e.g., high load compositions, which provides the maximal dense particle to particle contact of the thermally conductive particles to minimize interface thermal resistance for forming a good thermal path. However, such high load compositions do not always yield physical properties suitable for every end-use application.
For instance, high load compositions typically have a high viscosity, e.g. 200,000-300,00 cps at 25xc2x0 C., which results in poor flowability during application. Additionally, high load, high viscosity compositions may have a high thermal conductivity often resulting in thick bond lines which reduce the efficiency of the heat transfer. Increasing pin counts in smaller amounts of space demands a thermally conductive composition with a lower viscosity to enable thin, precise lines to be formed on the micro-sized semiconductors, thereby allowing for the development of efficient miniaturized parts. Thermally conductive compositions with a low viscosity are also desirable for the new, more powerful and denser semiconductor chips currently being designed because low viscosity compositions have better flowability. Low viscosity, thermally conductive compositions, such as those having a viscosity under 80,000 cps at 25xc2x0 C., are also particularly desirable for fragile chips and solder bonds that attach the chips to a substrate because the minimal force necessary to apply the adhesive to the semiconductor chip is reduced, thereby decreasing the risk of compromising the integrity of a fragile chip.
To reduce the viscosity of conventional thermally conductive compositions, lower loads of the thermally conductive component may be used, however, this act also reduces the thermal conductivity of the composition. Lower load compositions also may exhibit increased moisture absorbency, which may cause the semiconductor assembly to crack when heated due to the rapid expansion of internal moisture. This may result in disruption of the connectivity of the conductive particles, causing a disconnect in the microelectronic device in which the semiconductor is used.
Additionally, reducing the load of the thermally conductive component may also increase settling of the conductive particles over time and during curing. The tendency of the thermally conductive particles to settle out during heat cure is of particular concern. Heat cure causes the viscosity of the composition to decrease, allowing the thermally conductive particles to settle to the bottom of the adhesive film. The top layer, if deficient of thermally conductive particles, becomes thermally insulating and reduces thermal conductivity of the film. Thus, current high load and low load thermally conductive compositions are less than satisfactory.
Accordingly, it would be desirable to provide a thermally conductive composition is highly conductive and has a low viscosity providing excellent flowability.
The present invention provides a thermally conductive composition which, due to the incorporation of spherical thermally conductive particles therein, exhibits excellent flowability. The cured inventive compositions are highly conductive and have enhanced mechanical, moisture resistance and chemical resistance properties. The compositions of the present invention may be used as adhesives, sealants as well as coatings and the like. For purposes of this invention, the term xe2x80x9csealantsxe2x80x9d will include all such uses.
In one aspect of the invention there is provided a low-viscosity, thermally conductive composition which includes a curable resin component, a curing agent for the curable resin component, and a thermally conductive component which includes spherical thermally conductive particles substantially uniformly throughout the composition. A predominant amount of the spherical particles, and desirably at least 90% of the spherical particles, have an average diameter sufficiently low to maintain the particles in suspension. Desirably the spherical, thermally conductive particles are spherical alumina particles. The low-viscosities of the present invention are desirably between about 10,000 to about 80,000 cps at 25xc2x0 C.
Another aspect of the invention includes the reaction product of such a low-viscosity, thermally conductive composition.
Also contemplated by the present invention is an article of manufacture, which includes the inventive flowable thermally conductive composition stored in an openable container for packaging the flowable thermally conductive adhesive composition of the present invention.
Another aspect of the invention provides an electronic part encapsulated by the inventive composition. For example, such parts may include a wiring circuit board and a semiconductor chip mounted through electrodes to the circuit board and a space between the circuit board and the semiconductor. The periphery of the semiconductor chip is desirably encapsulated with the inventive thermally conductive compositions. Also contemplated are methods to encapsulate an electronic part, including the steps of providing the inventive thermally conductive composition and applying an effective amount of the composition to the electronic part to thereby encapsulate the electronic part.
Yet another aspect of the invention relates to a method of manufacturing a thermally conductive composition. Such a method includes combining in admixture at least one curable resin component with at least one curing agent for the curable resin component and a thermally conductive component. The thermally conductive component includes spherical thermally conductive particles. A predominant amount, and desirably at least 90% of the spherical particles, have an average diameter sufficiently low to maintain their suspension in the low viscosity sealant composition.
Yet another aspect of the invention relates to a thermally conductive, low viscosity composition which includes (a) a resin component; (b) a curing agent for the resin component; and (c) a combination of thermally conductive particles, which desirably are spherical alumina particles and a suspension aid for maintaining the conductive particles in suspension, i.e. improving their resistance to settling-out. The suspension aid is desirably zinc oxide. This suspension aid is particularly useful in preventing the tendency of the thermally conductive particles from settling-out, e.g. particularly during heat cure of the resin. Such settling out can be substantially decreased or prevented by the inclusion of a suspension aid.